you dont' pay tax on the first £10,600. So about 20% on £1400, which is nto a lot (but I'm no expert so others may be able to give you more precise advice)
Very little, if any. the tax free allowance is £10600pa. Yor tax though will depend on the earnings you've already had this tax year. If none then you won't have any tax to pay this tax year
Assuming that you've got no other taxable income, you'd pay hardly any Income Tax.
The first £10600 of income in a year is free of tax. So you'd only pay tax on £1400 (at 20%). So that's £280 tax to be paid across the year, which is equivalent to about £5.38 per week or £23.33 per month.
However you'd also have to pay National Insurance (which is just a tax under a different name). Expect to pay about £9.09 per week or £39.40 per month.